linux-kernel - Re: [PATCH 2/3] riscv: Set dma_default

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Message-Id: <6BBA7BEA-8595-436D-B4BF-D7DB95069C53@flygoat.com>
Date:   Wed, 22 Feb 2023 15:55:19 +0000
From:   Jiaxun Yang <jiaxun.yang@...goat.com>
To:     Conor Dooley <conor@...nel.org>
Cc:     "linux-mips@...r.kernel.org" <linux-mips@...r.kernel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Thomas Bogendoerfer <tsbogend@...ha.franken.de>,
        mpe@...erman.id.au, paul.walmsley@...ive.com, palmer@...belt.com,
        Rob Herring <robh+dt@...nel.org>,
        Christoph Hellwig <hch@....de>, m.szyprowski@...sung.com,
        robin.murphy@....com, linux-riscv@...ts.infradead.org
Subject: Re: [PATCH 2/3] riscv: Set dma_default_coherent to true



> 2023年2月22日 14:50，Conor Dooley <conor@...nel.org> 写道：
> 
> On Wed, Feb 22, 2023 at 01:37:11PM +0000, Jiaxun Yang wrote:
>> For riscv our assumption is unless a device states it is non-coherent,
>> we take it to be DMA coherent.
>> 
>> For devicetree probed devices that have been true since very begining
>> with OF_DMA_DEFAULT_COHERENT selected.
>> 
>> Signed-off-by: Jiaxun Yang <jiaxun.yang@...goat.com>
>> ---
>> arch/riscv/kernel/setup.c | 3 +++
>> 1 file changed, 3 insertions(+)
>> 
>> diff --git a/arch/riscv/kernel/setup.c b/arch/riscv/kernel/setup.c
>> index 376d2827e736..34b371180976 100644
>> --- a/arch/riscv/kernel/setup.c
>> +++ b/arch/riscv/kernel/setup.c
>> @@ -300,6 +300,9 @@ void __init setup_arch(char **cmdline_p)
>> riscv_init_cbom_blocksize();
>> riscv_fill_hwcap();
>> apply_boot_alternatives();
>> +#ifdef CONFIG_RISCV_DMA_NONCOHERENT
>> + dma_default_coherent = true;
>> +#endif
> 
> Do we really need to add ifdeffery for this here?
> It's always coherent by default, so why do we need to say set it in
> setup_arch() when we know that, regardless of options, it is true?

Because this symbol is only a variable when:

defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_DEVICE) || \
defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU) || \
defined(CONFIG_ARCH_HAS_SYNC_DMA_FOR_CPU_ALL)

Which is only true if  CONFIG_RISCV_DMA_NONCOHERENT is selected.

Otherwise this symbol is defined to true and we can’t make a assignment to it.

Thanks
- Jiaxun


> 
> Cheers,
> Conor.
> 
>> if (IS_ENABLED(CONFIG_RISCV_ISA_ZICBOM) &&
>>     riscv_isa_extension_available(NULL, ZICBOM))
>> riscv_noncoherent_supported();
>> -- 
>> 2.37.1 (Apple Git-137.1)